Summary: Ammonium-grown cultures of developed the capacity to assimilate nitrate when they were incubated in nitrate or nitrogen-free medium. Cycloheximide, 6-methyl purine and antimycin A inhibited this process. Nitrate assimilation required aerobic energy metabolism and was inhibited completely by ammonium. The rapid inhibition of nitrate assimilation by ammonium was not the result of an inhibition of nitrate reductase (NR) activity. Nitrite also inhibited nitrate assimilation. NR in cell-free extracts of was NADPH-specific and its activity was repressed in cultures containing ammonium and derepressed during nitrogen starvation. Nitrate stimulated the appearance of NR in these cultures. Nitrate assimilation was not limited by apparent (potential) NR activity.


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